This invention relates to a control system in a stepless transmission, especially the transmission of an automotive vehicle. More particularly, the invention relates to a control system used in controlling a stepless transmission of the type comprising, in combination, a stepless transmission unit using a belt (where the meaning of the term "belt" is taken to cover a chain) and an auxiliary transmission unit, such as a planetary gear unit, for widening the torque, ratio range.
Because of to the demand for improved fuel consumption in automotive vehicles, stepless transmissions which incorporate a stepless belt transmission unit (CVT) have become the focus of attention. In general, a stepless transmission is composed of a stepless belt transmission unit, a fluid coupling (or an electromagnetic powder clutch), a forward/reverse changeover unit, a speed reducing gear unit and a differential gear unit. As a consequence of such limitations as space and the minimum radius of curvature of the belt, the abovementioned stepless belt transmission unit is incapable of providing a large torque ratio range. The requirements for improved fuel consumption and shifting performance in an automotive vehicle cannot be fully satisfied within this torque ratio range provided solely by the stepless belt transmission unit.
As set forth in the specification of Japanese Patent Application Laid-Open No. 61-31752, a stepless transmission has been proposed in which an auxiliary transmission unit comprising a Ravigneaux-type planetary gear mechanism is serially connected with a stepless belt transmission unit, and the auxiliary transmission unit is changed over between low- and high-speed stages to broaden the torque ratio range.
In this stepless transmission, switching the auxiliary transmission unit to the high- and low-speed stages is performed by shifting a shift lever to an L (low) range and a D (drive) range. The stepless transmission unit is suitably controlled so as to attain a target engine speed (rpm) decided by the vehicle speed and throttle opening. The target engine speed is set separately for each of the aforementioned shift positions, namely for the low-speed stage and high-speed stage of the auxiliary transmission unit.
However, since the auxiliary transmission unit is changed over to the low- and high-speed stages by operating the shift lever, the abovementioned stepless transmission is troublesome to manipulate and difficult to operate properly.
Accordingly, the applicant has previously proposed a stepless transmission control system (see Japanese Patent Application No. 62-99360, filed in Japan on Apr. 21, 1987) in which the stepless transmission unit and auxiliary transmission unit are suitably controlled, based on predetermined decisions, with respect to a target torque ratio decided by such traveling conditions as engine speed and throttle opening, whereby the auxiliary transmission unit is changed over to the low-speed mode and high-speed mode automatically.
When control is exercised to halt the vehicle, and in particular, when control is performed in accordance with an optimum fuel consumption characteristic in the abovementioned control system, control is carried out in such a manner that the target engine speed is reduced, thereby lowering the torque ratio, when the driver's foot is removed from the accelerator pedal. Furthermore, control is such that when coasting continues or the brake is operated, vehicle velocity decreases and torque ratio gradually rises, with the maximum torque ratio being attained when the vehicle comes to rest. This is necessary for the vehicle to start out smoothly the next time. In particular, to protect the belt in a stepless belt transmission unit, a shifting operation cannot be performed while the vehicle is at rest. Consequently, it is required that the stepless transmission unit be controlled so as to obtain a maximum torque ratio in a reliable manner.
In an arrangement wherein the auxiliary transmission unit is changed over such that operation in the high-speed mode is given precedence when the target torque ratio is in a region (see B in FIG. 36) where both the low- and high-speed modes can be achieved equally and in a region (see C in FIG. 36) where only the high-speed mode can be achieved, and such that operation in the low-speed mode is given precedence when the target torque ratio is in a region (see A in FIG. 36) where only the low-speed mode can be achieved, the stepless transmission unit is shifted down (see arrow E in FIG. 36), so that the unit attains the maximum torque ratio, when the high-speed mode is in effect at the time control is executed to stop the vehicle. After the maximum torque ratio is achieved, the auxiliary transmission unit is changed over to the low-speed mode, and the stepless transmission unit is upshifted (see arrow F in FIG. 36) in order that its torque ratio will approach the maximum torque ratio of the high-speed mode. Thereafter, the stepless transmission unit is downshifted as vehicle speed declines.
However, when the auxiliary transmission unit is changed over from the high-speed mode to the low-speed mode, the stepless transmission unit is upshifted and then the vehicle is suddenly reduced in speed and stopped by applying the brake, there is the danger that the downshift of the stepless transmission unit will not occur in time so that the maximum torque ratio will not be achieved when the vehicle stops. When this phenomenon occurs, there is not enough torque when the vehicle is propelled forward the next time; hence, performance at start-off declines.
Accordingly, in the previously filed application mentioned above, a target torque ratio a of the overall stepless transmission is set, a mode changeover means is changed over in such a manner that the high-speed mode H operates when the set target torque ratio is in the region B, where both the low-speed mode L and high speed-mode H can be achieved equally, and such that the low-speed mode L operates when the set target ratio is in the region A. Here the stepless transmission unit is held at a maximum torque ratio a.sub.H max, without effecting a changeover from the high-speed mode H to the low-speed mode L, while the stepless transmission unit is upshifted (see arrow F) from the maximum torque ratio a.sub.H max of the high-speed mode H.
In this control system, however, even if the target torque ratio at the time of deceleration is a torque ratio in which only the L mode is attained, a changeover to the L mode must be made if the accelerator pedal is depressed when a shift to the L mode is being halted. At this time there is a rapid change in torque ratio which detracts from the feeling of smooth travel. On the other hand, if the changeover from the H mode to the L mode is delayed, a low torque ratio will be established. As a result, no driving force will be obtained or knocking will occur.
In a conventional control system as disclosed, for example, in Japanese Patent Application Laid-Open No. 59-212566, the shifting speed of a stepless transmission mechanism is set to a larger value, in dependence upon vehicle speed, the greater the throttle opening of the intake system or the greater the rate of change in throttle opening with time, thereby making it possible to prevent a deterioration in accelerability and fuel efficiency. In this control system, however, there is no change in throttle at a throttle opening .theta.i of .theta.min.gtoreq..theta.i (where .theta.min is the minimum set value of throttle opening), so that the shifting speed at the time of coasting is set to a minimum value. Though shifting speed is satisfactory with regard to a change in target torque ratio accompanying a decrease in vehicle speed, a downshift cannot catch up with a sudden change in target torque ratio when the brake is suddenly applied, and there is the possibility that the belt of the stepless transmission mechanism will not downshift completely when the vehicle comes to rest.
In a stepless transmission proposed in Japanese Patent Application Laid-Open No. 62-13853 as shown in FIG. 35, the rotation of a motor operatively associated with a worm gear 67 is controlled, based on vehicle travel signals indicative of vehicle speed, throttle opening, engine speed and the like, when a shift is made. For example, when the worm gear 67 is rotated in the upshift direction (clockwise in FIG. 35), a worm wheel 66 rotates in the clockwise direction as seen from the right side of the drawing (directions of rotation will be as seen from the right side of the drawing hereinafter) so that gears 61, 62, 63, 65 are also rotated in the same direction via a counter shaft 57. When this occurs, gears 26, 27 of a ball screw mechanism 21 on the primary side rotate counterclockwise. Based on a difference in the number of teeth, the rotational speed of the small-diameter gear 27 is greater than that of the large-diameter gear 26. Consequently, a male screw portion 23 that rotates together with the gear 27 via a ball spline 29 undergoes greater rotation than a female screw portion 22 that co-rotates with the gear 26. A ball screw, which comprises a right-handed screw, is caused to extend in the rightward direction so that a movable sheave 7 is moved via a thrust bearing 31 so as to reduce the gap between itself and a stationary sheave 9, thereby enlarging the effective diameter of a belt B. Similarly, with regard to gears 49, 47 of a ball screw mechanism 50 on the secondary side relying upon gears 63, 65, a male screw portion 46 that co-rotates with the gear 49 undergoes greater rotation than a female screw portion 45 that co-rotates with the gear 47 based on the relative rotation in the counterclockwise direction. As a result, a ball screw comprising a right-handed screw is withdrawn in the rightward direction to move a movable sheave 33 so as to reduce a gap between it and a stationary sheave 35, thereby reducing the effective diameter of the belt B.
In this conventional stepless transmission, however, a speed reduction from a motor serving as the shift actuator is made at the worm gear, which has a poor transfer efficiency. Consequently, a motor of a fairly large capacity relative to the actual load capacity is required. In addition, in a case where a combination of a high-efficiency spur gear and brake is employed instead of the worm gear, there is a possibility that the next shift command will arrive as soon as a speed change in the stepless transmission ends, so that it is difficult to make a decision regarding application of the brake. In other words, if the brake were to be frequently applied every time a shift ends, the durability of the brake would suffer.
In accordance with another control system, the gear ratio is subjected to downshift or upshift control, based on a preset shift control characteristic, in such a manner that the input rotational speed is made to correspond to the engine throttle opening. By virtue of this control, the engine is operated at a constant torque and constant output, output is increased and fuel is consumed more efficiently. Also, in Japanese Patent Application Laid-Open No. 62-166120, a system is disclosed in which, when a vehicle is decelerated, the control gain of downshift control with regard to the gear ratio of the vehicle is revised to be larger than what prevails normally, thereby raising control speed in the downshift region of the gear ratio and enhancing the engine braking effect at deceleration of the vehicle. At subsequent re-acceleration, a response delay is eliminated by an enlarged torque transfer ratio, there raising acceleration response.
However, when it is attempted to suddenly decelerate and stop the vehicle by an operation such as sudden braking, a problem that arises is that the downshift of the stepless transmission mechanism is not in time and the mechanism is fixed at an intermediate torque ratio with respect to the maximum torque ratio. When it is attempted to start the vehicle moving forward again in such case, the maximum torque ratio is not achieved, as a result of which the stepless transmission mechanism undergoes a violent downshift at forward motion of the vehicle. Consequently, forward motion of the car is accompanied by a shock.
In the stepless transmission proposed in the aforementioned Japanese Patent Application Laid-Open No. 62-13853, rotation of the motor coupled to the worm gear is controlled based on vehicle travel signals indicative of vehicle speed, throttle opening and engine rpm at the time the transmission is shifted. In this stepless transmission, however, the stepless transmission mechanism will downshift and, in some cases, overrun, if the accelerator pedal is depressed when the motor or the driver controlling the motor malfunctions to place the motor in an uncontrollable (free) or runaway state. On the other hand, if the accelerator pedal is suddenly released from an accelerator opening of .theta., the stepless transmission mechanism will undergo an upshift and it will not be possible to the vehicle to travel normally. If control of torque ratio thus becomes impossible, it is imperative that the torque ratio be fixed. However, in a stepless transmission mechanism, the target torque ratio frequently changes in response to throttle opening and the motor cannot immediately catch up with the target torque ratio. Consequently, there are instances in which there is a large deviation between a commanded value and a currently prevailing value, even when operation is normal.